Automatic audio tone control circuit



Nov. '18, 1941. E. w. HEROLD AUTOMATIC AUDIO TONE CONTROL CIRCUIT FiledSept. 15, 1939 AMPLIFIER 7'0 UTILIZATION MEAN-5' A UD/O KOLTAGE SOURCEOF I000 FREQUENCY (0 m1. E6

INVENTOR. EDWAR? [IE/20. .0

7K Ad-wb ATTORNEY.

Patented Nov. 18, 1941 Edward W. Herold, Verona, N. Y., assig'nor toRadio Corporation of America, a corporation of Delaware ApplicationSeptember 15, 1939, Serial No. 295,033

3 Claims.

My present invention relates generally to arrangements for automaticallyvarying the audio frequency response characteristic of an audiotransmission system, and more particularly to an improved method of, andmeans for, automatically varying the transmitted audio band Width inresponse to the amplitude level variation of audio input energy.

Communication systems for the transmission, or reproduction, of audiblesound are usually limited in volume range by background noise. It iswell known that the annoyance caused by an audible fluctuation noise isgreater when the noise contains the higher frequencies of the audiblespectrum. This is true because, although the noise energy is uniformlydistributed over the frequency spectrum, the useful sound energy ismainly concentrated in the lower frequencies. Thus, the highesteffective signal to noise ratio in the presence of disturbing noise isusually obtained by cutting down the highest frequency which can betransmitted. Accordingly, the scratch noise present during playing of anordinary phonograph record is less annoying when the reproduced audiofrequency band does not extend above substantially 3,000 cycles; In highfidelity musical reproduction from records, this difliculty can only beovercome by recording at a sufficiently high level so that the weakestpassages of music are suflicient to override the noise. This is notalways possible, however, particularly when the music, has loud passageswhich it is Accordingly, it may be stated that it is one of the mainobjects of my invention to provide an improved method of, and means for,automatically controlling the band width of the transmitted audiosignals by a special regulating device whose function it is to adjustthe fremission channel is capable when the level of desired sound energyis above a predetermined value which is sufficiently great substantiallyto -eliminate annoyance from background noise;

desired to reproduce without over-cutting the and the controladditionally reduced the frequency spectrum to an extent sufficient suchthat undesired noises become less objectionable in the output of thegain transmission channel as soon as the desired sound amplitude levelfalls below a certain minimum and background noises becomes noticeablygreat.

Still other objects of this invention are to improve the operation ofautomatic arrangements for regulating the high audio frequency responseof an audio signal transmission system,

the control arrangement including an audio signal rectifier whichfunctionsto varythe gain of an electron discharge tube having circuitsasprove generally automatic audio frequency refrequency width of theaudio system is in use for high fidelity reproduction. However, as soonas the desired sound energy decreases substantially to noise level, thewidth of the audio frequency spectrum should automatically be reduced toa value giving a higher effective signal to noise sponse controlcircuits, and more especially to provide an audio signal transmissionsystem with means for regulating the high audio response thereof, andwhich control means is reliable in operation and is economicallyassembled in an audio transmission system.

The novel features which I believe to be characteristic-of my inventionare set forth in particularity in the appended claims: the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organiza condenser l l transmits pairofanodes.

reproducing phonograph records, or it may be the detector output circuitof a radio receiver of the'broadcast type. It can even be the microphoneof a public address system, or the photoelectric-pick -up device usedwith talking moving pictures. In general, then, the source I is to beunderstood as feeding to the potentiometer re sistor 2'audio signalvoltage covering a range of approximately zero to 15,000 cycles, which.latter range is generally considered as being the high fidelity audioreproduction range.

The numeral 4 denotes an audioamplifier tube mnemoni or type. Forexample, it may be the. ei ec 1 trical pick-up device used in connectionwith veloped across resistor i9 is impressed on the inputpgrifaipf thecontrol tube 22. The control voltage is inipressed on grid 2| through apath which includes the filter resistor 23 and the grid leak resistor24, and the junction of the resistors is grounded througha condenser 25.f The resistors i9 and Hand condensers 20 and 25 cooperate to provide aresistance-capacitance filter,

of any well known type, and whose signalinput grid 5 is connected bysource of audio voltage I. v The cathode-oi tube '4 is connected togroundthroughthe usual resistor-condenser grid biasing network 6.v Thescreen grid of tube 4 has'positive voltage applied 'to' it' through theusual resistor I.the latter'being by-passed to the cathode-byacapacitor. The

the adjustable tap 3 to the" potentiometer resistor 2 shunted across theible a full waverectifier is employed time constant needed is only halfthat which would be required with a half wave rectifier. .Of course,halt wave rectification can be used in' tential for plate and the'timeconstant ofthis filter should be Just long enough to prevent lowfrequency alternating voltage from being applied to the grid 2|. Inorder to make the deviceoperate as rapidly as posso that the such caseswhere cost considerations make a slight sacrifice in performancedesirable. The tube 22 has circuits associated therewith such that thetube functions as an electronic capacitance. Theplate 26 of the controltube is connected to thelplate end of resistor '9 through a blocking.condenser 21, while the cathode of the tube is connected to groundthrough the resistor 28. The positive po- 26 is applied through a pathwhich includes the coil L and resistor R. A capacitance 29 is connectedbetween grid 2| and the junction of resistor R and coil L, and thecapaciusual self-biasing tance'iunctions as a blocking condenser. Gridleak resistor 24 is a high resistance resistor.

'- Considering the control. tube22 more specifically suppressor grid-oftube 4 is at groundpotential,

while the plate 8 is established at adesired positive potential by theload resistor 9.. Numeral ll designates schematically a succeeding audioamplifier network, and the latter-may comprise one or more audioamplifier tubes. The coupling The amplified audio voltageoutput oiamplifier I!) may be impressed upon any desired utilization means, suchvas a reprpducer or the loudspeaker type. The high audio frequencyresponse oi the audio response characteristic is automatically varied bythe control channel which includes an amplifier l2. It is to beunderstood that this tubes, or, if desired,

\ the audio voltage devel- 'oped across'load resistor Sto the inputelectrode it is to be noted that the' gri'd 2| 'is' connected to plate26 through the relatively high resistanceR. while inductance 1 connectsthecontrol grid to the cathode: If an alternating voltageis appliedacross the plate to cathode path, and if resistor R is sui'flcientlyhigh, a. current will flow through and L which is substantially in phasewith the applied voltage. The voltage drop across coil L is applied tothe currentand thusthe applied voltage) by approximately 90 degrees.

Theresulting alternating current plate current due to this grid voltagewill then lead the applied voltage by about 90 degrees. Assuming R is soamplifier may include one or more amplifier the amplifier-may beomitted.

A desired value of audio signal voltage is im- 9 pressed on amplifier 12by providing in shunt with resistor 2 a resistor l3. The adjustable tap5 l4 enables the operator to select the desiredv ma nitude of audiosignal voltage which is to be employed for the control function. Inother words, taps 3 and provide independent means for permitting theregulation or the magnitudes of the input audio. signal voltage and thecontrol audio voltage. This is o! decided advantage in providingincreased flexibility of control. The output'of amplifier I2 isimpressed-upon a full wave rectifier which comprises. the double diodetube I5 having a commoncathode and a One of the anodes I5 is connectedto one end of the secondary winding of 7 audio transformer", while theanode I8 is connected to the opposite end of the. said winding. Thegrounded cathode of tube I5 is connected to the mid-point of thesecondary winding through a load resistor l9 which is appropriatelyby-passed by condenser 20. The uni-directional voltage dehigh that thecurrent through it can be neglected, the alternating" voltage appliedbetween plate and cathode of tube 22 results in a current across'theseelectrodes which leads the voltage by approximately degrees. In otherwords, the impedance between the plate and cathode of tube 22 functionsin the manner of a capacitance, and

the magnitude of this capacitance is proportional to the alternatingplate current. Thus, when 1 the .transconductance of tube 22 is variedas by varying the bias ofgrid2l, the eflective capacitance between the"cathode and plate is varied.

- It can be demonstrated that a-variation or the Fur of tube 22 willvary the effective capacitance provided by the impedance betweenthe'cathode and plate of tube 221almost linearly,- thus justifying theconcept that the'tube22 behaves as a variable capacitance. v I v -To"usetube 22 for variable high frequency audio cut-oii the plate to cathodeimpedance of the tube is connected across the resistive plate load 9 ofamplifiergtube 4. In Fig. 2 there is shown various frequency responsecurves secured with different values of transcpnductance for tube 22. Insecuringthe various curves in Fig. 2 it was assumed that the resistiveplate load of the amplifier 4 is of the order of 100,000 ohms, and invconsideringthis magnitude the plate resistances of tubes4 and 22am tobe included in thisyalue.

grid'2l and is then leading the The resistor R had a value of 200,000ohms, while coil L had a value of 0.5 henry; it was considered that coilL had internal losses corresponding to a series resistor value of 200ohms, and that am of tube 22 had a value of 2X mhos maximum. Aninspection of the curves of Fig. 2 will show that the Irequency responseis varied from the uppermost fiat curve, with a value of cm which equalszero, to the lowermost of! around 1,000 cycles. Also, it will beobserved that the high audio frequency response depends directly on thetransconductance of the tube 22. The control grid bias of tube 22 isvaried without the consumption of power, and, therefore, represents adesirable manner of controlling the frequency response of the audiosignal transmission channel. a c

In considering the operation of the circuit arrangement shown in Fig.'1, the audio input voltage 'isapplied at the. adjustable potentiometer2+4 and the amplified, audio voltage is transmitted to the utilizationmeans as described above. The frequency response of the maintransmission channel will be determined by the response characteristicof plate load resistorv 9 and the shunt admittance connected across itby means of blocking condenser 21 and tube 22 and its associatedcomponents. The audio input signal is simultaneously applied to theadjustable potentiometer I2l| which may be'adjusted for selectingthescratch amplitude level which is decurve which cuts tion shown byFig. 2 is accompanied by a very slight volume expansion action. For lowlevels of the incoming sound, the relative maximum gain is slightly lessthan for high levels of the incoming sound. This behaviour is caused bythe internal losses in the coil L together with the fact that theresistor -R is not infinitely larger than the reactance of L. As aresult the tube 22 and its accompanying circuit does not behave as apure variable capacitance, but it also behaves as a variableconductance. The circuit values which were used for the illustrativecurves ,of Fig. 2 were chosen, for simplicity, so that the volumeexpansion action was very small. By increasing L and decreasing R, or byinserting additional resistance in series with L, the volume expansionaction .may be made appreciable.

Such behaviour is of advantage when the volume range of the incomingsound has been restricted.

since it tends to restore the original volume range.- The same actionalso tends to minimize sired to be tolerated. After implificationatamplifier l2 and rectification at double diode l5,

the control voltage is applied to the grid 2| of the tube 22. a

With noises, such as scratch noises, only impressed at the input networkof the system, tap I0 is adiusted upwardly along resistor i3 until tube22'just begins to decrease in plate current.

Under these conditions tube 22 is operated at approximately maximumtransconductance, and

the response. curve is similar to one of the lower curves shown in Fig.2. The scratch components in the audio output of amplifier iii are,therefore, less annoying because of the absence of high audio frequencycomponents. When the audio signal amplitude exceeds the scratch, ornoise, level slightly, the control grid 21 is biased more negatively andthe response curve of the system approaches one of the higher curves ofFig. 2. For audio signal amplitudes well above the scratch level audioreproduction is in accordance with the uppermost curve of Fig. 2, sincein that case tube 22 is completely cut-off. In this event full highfidelity reproduction is available. It will, therefore, be seen that theaudio control channel provides a means for utilizing a unidirectionalvoltage derived irom the audio signal itself, and which uni-directionalvoltage is employed to vary the gain oi a tube whose plate tocathode'impedance functions as a capacitance connected in shunt acrossthe main audio signal transmission channel.

In some cases it may be advantageous to use a delayed bias on the doublediode It. Again, it is not essential to the present invention that theinput to the control channel be derived from across source i, since theinput to the control channel may be taken from the plate resistor 9 oreven from across the output of amplifier 10. Additionally, the variableelectronic capacitance may be connected at other points of thetransmission channel for varying the response characteristic.

nois'e'and scratcli'which occur at low sound levels since not only isthe frequency range reduced by the shunting capacitance action of tube22, but the gainat all frequencies may be reduced by the shuntconductance action of the same tube.

While I' have indicated and described a system for carrying my inventioninto efl'ect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In an audio transmission system of the type including at least oneaudio amplifier tube and a utilizing means, a frequency response controlarrangement including an electron discharge tube having circuitsassociated therewith so as to render the plate to cathode impedance ofthe tube operable as a reactive impedance, means for connecting saidreactive impedance across the output of said amplifier tube, and meansconnected across the amplifier tube input for deriving a uni-directionalcontrol voltage from audio voltage impressed upon said amplifier tube,and means for impressing said uni-directional voltage upon a controlelectrode of said control tube thereby to vary the magnitude of saidreactive impedance.

2. In combination with a main audio voltage transmission channel, acontrol tube having its plate to cathode impedance connected across thechannel to function as a shunt reactive path, and means connected acrossthe main channel input and directly responsive to amplitude of the audiosignal voltage impressed upon said main channel for controlling the gainof said control tube thereby to control the magnitude of said shuntreactive path.

3. In an audio voltage transmission system ofand control grid, aconnection between the out-' put electrode of the control tube and saidaudio transmission system for impressing audio volt- It should, also, bepointed out that the operaage between the cathode and output electrode

